The present invention relates to a device for degassing molten steel with an improved outlet nozzle. In particular, the present invention relates to a special shape of an outlet nozzle for avoiding local dead water regions in a steel casting ladle. The present invention further relates to a method for degassing liquid steel with the improved outlet nozzle.
The method for degassing liquid steel is an RH-method (Ruhrstahl-Heraeus method). In the RH-method, the liquid steel is conveyed from a casting ladle in a riser pipe into an evacuation vessel. A conveying gas, in particular argon, is introduced into the riser pipe above the level of the steel bath. The argon flow introduced into the riser pipe through several nozzles, disintegrates into a plurality of argon bubbles which rise in the immediate vicinity of the wall. The conveyance of the liquid steel is facilitated by the volume enlargement because of argon in the riser pipe and by the pressure difference between the outer air pressure and the negative pressure in the evacuation vessel. The argon bubbles entrain the molten steel and ensure a uniform circulation of the molten steel. The partial pressure is simultaneously lowered and the decarburization reaction is accelerated. The steel taken into the evacuation vessel is sprayed. As a result, a significant surface increase and a good degassing of the liquid steel occur.
Oxygen, which during the entire treatment time is taken in simultaneously and is, among others, supplied from the slag, leads to the formation of carbon monoxide (CO). CO is degassed in the vacuum vessel, so that the desired decarburization is achieved. The fine decarburization to values which are as low as possible can be optimized by oxygen which is additionally blown in. A high circulating speed of the molten steel and, thus, an increase of the conveying gas flow and an increase of the nozzle diameter of the vacuum plant, lead to a faster decarburizing sequence.
DE 19511640 C1 discloses a nozzle for a degassing vessel with a refractory lining and a gas rinsing device with several ducts arranged in the lining. The ducts are distributed over the circumference of the nozzle and extend, in relation to the center longitudinal axis of the nozzle, through the refractory lining in a radial direction. The ducts can be connected at the outer side to at least one gas supply line.
For forming an almost continuous gas veil, the ducts are arranged in close sequence circumferentially along the inner wall of the nozzle. A uniform flow of liquid steel is achieved up to and into the vacuum vessel. The gas supply which is distributed over the entire circumference facilitates, preferably through fine bubbles, an especially fine distribution of the treatment gas with a simultaneously significantly increased reaction volume between treatment gas and molten steel. In this manner, a higher and faster decarburizing output is achieved, so that smaller quantities of reduction media are necessary.
JP 6299227 A discloses a method for manufacturing steel with very low carbon content by means of a degassing device, wherein the inlet nozzle is positioned such that the distance between the axis of the inlet nozzle and the axis of the metal bath is at least 10% of the inner diameter of the metal bath.
JP 1198418 A discloses a device and a method for vacuum degassing of molten steel, wherein gas is introduced into the inlet nozzle and the outlet nozzle, and the function of the nozzle can be alternated.
JP 57200514 A discloses a method for degassing molten steel, wherein the degassing effect is improved by degassing an RH-vacuum apparatus, in which an inert gas is blown into a molten steel vessel from the bottom.
JP 3271315 A discloses an RH-vacuum decarburizing method of noble steel, wherein degassing and decarburizing are achieved in a short time and the chromium loss is reduced. The result is achieved by using steel having a low silicon content and by repeated degassing and decarburizing procedures with an RH-vacuum vessel.
JP 2173204 A discloses a vacuum vessel for an RH degassing device, wherein an ultrasound oscillator is mounted at a contact point with the liquid steel in the vacuum vessel, for destroying bubbles which are produced by the blowing in of gas, and for improving the reaction surface at the phase reaction.
JP 11158536 A discloses a method for melting steel having a very low carbon content, wherein an inert gas is blown through the inlet pipe below the added aluminum into the vessel at the outlet nozzle for circulation after decarburizing.
JP 3107412 A discloses a method for manufacturing steel with a very low carbon content, wherein during decarburizing, argon is blown simultaneously into the inlet as well as the outlet pipe.
It has been found, and is confirmed by numeric simulations, that in the steel casting ladle of an RH plant local flow regions, so-called dead water regions, are formed which are mixed relatively late, only after about two minutes, with the remaining molten steel.
The devices and methods known in the prior art have the disadvantage that dead water regions are formed in the steel casting ladle which increase the homogenization time of the molten steel.
A dead water region is usually formed between the outlet nozzle and the refractory wall of the casting ladle. Through the downwardly directed jet of molten steel from the outlet nozzle, a small quantity of material is taken in from the direct surroundings around the outlet nozzle. Consequently, because of the delayed homogenization, the carbon concentration remains altogether at a high level at this location. The dead water region mixes poorly with the remaining molten steel because the average flow velocity is low. Because of the low exchanges of mass, pulse and energy between the dead water region with high carbon concentration and the remaining molten steel with low carbon concentration, the molten steel in the ladle must be frequently circulated until the desired final carbon content is achieved. Since the molten steel in the ladle must circulate frequently, the treatment time is long.